1. Field of the Invention
The invention relates to a pressure sensitive adhesive transfer tape and more particularly to a transfer tape having a carrier web and recesses in the web for the adhesive. The invention also relates to patterned adhesives and a method for forming and maintaining a patterned pressure sensitive adhesive until said adhesive is transferred to an article.
2. Description of Prior Art
Pressure sensitive adhesive (PSA) transfer tapes or double stick tapes find wide application in bonding two substrates or surfaces together because of the advantages offered over dispensing and applying adhesives from a tube or container. In using transfer tapes, it is often desired to transfer adhesive to the smaller of the two surfaces, such as a part, component or fastener, being bonded. Often, the objective is to cover as much of this surface with adhesive, so as to enhance the bond, but not to have adhesive extending beyond the perimeter of this surface.
A common method for accomplishing transfer of adhesive is to die cut the adhesive or adhesive and liner of a transfer tape to the shape of said surface such that the adhesive just covers the smaller of the two surfaces being bonded. The die cutting approach is widely used in industry despite the cost and complications of cutting and indexing the adhesive to the part. Die cutting becomes more difficult as the part either increases in complexity or is significantly reduced in size.
An alternate approach is to formulate an adhesive that readily shears through its thickness. In this case the adhesive is coated onto a carrier film. When the surface of a part or component is applied to the adhesive coated carrier film and separated therefrom, the adhesive shears, that is, tears through its thickness, leaving the adhesive only on the surface of the part or component. Such a transfer tape is available from Minnesota Mining and Manufacturing Company (3M) under the designation "Transfer Tape Product 909." Such tapes are generally limited to applications not requiring high performance adhesion, for example, as an assembly aid for mechanical fasteners.
Another method for getting adhesive only on the smaller of the two surfaces being bonded, such as a part or component, is to divide the adhesive into segments on the carrier web. When a part is placed in contact with these adhesive segments and then separated, only the segments contacted by the part will be transferred to the part.
U.S. Pat. No. 3,741,786 (Torrey) and British patent publication 1384423 to Avery Corp. is illustrative of segmented adhesive art. In both of these patents, the adhesive is applied to a carrier by conventional means such as rotogravure printing, silk-screen printing, intermittent extrusion of an adhesive melt and the like as well as direct coating of an adhesive layer with subsequent cutting and stripping of an adhesive matrix from the transfer tape or web to provide the substantially noncontiguous raised pressure sensitive adhesive segments. Torrey also teaches that the raised adhesive segments may be dots, diamonds, stars, triangles, or mixtures thereof and that the segmented adhesive transfer tape may be used in an automated or manual dispensing apparatus. Other examples of segmented adhesive transfer tapes of the general type taught by Torrey are British Patent Specification 1420743 (Ghavt), French Application of Patent of Invention No. 81 11285 (Euverte), European Patent Application 0279579 (Tanuma et al.) and Japanese Patent Office Kokai Patent No. HEI 2(1990)--11684.
One of the shortcomings of such prior art segmented adhesive transfer tapes is that in forming the adhesive pattern on the carrier film, there is a tendency for many pressure sensitive adhesives to slump and flow laterally. This problem is exacerbated when patterned adhesive segments are either closely spaced or exhibit a high ratio of height to width. After depositing the adhesive pattern on a carrier web it is normal practice in the balance of the manufacturing, distribution and use of these tapes to stack sheets of the tape or wrap the tape into a roll. The force on the raised non-contiguous adhesive segments is such that the segments have a tendency to move laterally under cold flow conditions, that is, under ambient conditions, such that a continuous adhesive sheet is formed. The extent of this shortcoming is expressed in the above cited prior art. For example, U.S. Pat. No. 3,741,786 (Torrey) at col. 4 lines 1 to 10 states: "The pressure sensitive adhesive segments should not, however, be spaced so closely that prior to application to a substrate they will be compressed and thereby caused to form uniform adhesive layer which no longer has a definable zone of separation due to compressive forces resulting from manufacturing and/or handling. This would preclude removing the carrier tape from the applied pressure sensitive adhesive without tearing, rupturing, stretching, stringing or cutting the transferred pressure sensitive adhesive." British Patent Specification 1420743, p. 2, lines 115-121 states: "The pressure-sensitive adhesive should be of a character which is capable of maintaining its physical shape under normal conditions likely to be encountered during handling and storage. Pressure sensitive adhesives which slump or flow are unsatisfactory for use in accordance with the invention." European Patent Application 0279579, at p. 3, lines 11-15 states that islands or irregular sections of pressure sensitive adhesive on a substrate tend to change over time due to the fluidity of the adhesive. This reference teaches that to overcome this tendency, crosslinking structures should be introduced into the adhesive. Japanese Kokai Patent No. HEI 2(1990)11684 points out that a problem with segmented adhesives is that it starts to flow immediately after coating on a support substrate causing disfigurement of the coating pattern even before polymerization treatment.
In efforts to counteract these difficulties, practitioners in the art have made comprises relative to adhesive selection, the spacing between adhesive segments and the height of the segments. In addition, when the adhesive itself is modified or selected to prevent lateral flow during handling and storage under ambient conditions, it becomes more difficult to form a continuous adhesive bond after transfer. This is because the same adhesive behavior that prevents lateral flow during handling and storage prevents lateral flow after bonding and lateral flow after bonding is desirable since it improves the adhesive bond. When bonding small components, it is desirable to have the adhesive segments small because the larger the adhesive segments, the more the adhesive will extend beyond the perimeter of the part and ultimately will limit the size of the parts for which the tape is useful. European Patent Application 0382507 p. 2, lines 42-43 states that the need to have the adhesive segments small makes such transfer sheets unsuitable for use in adhering small items.
U.K. Patent Application GB 2058664A discloses a rigid wallboard which has been embossed and the raised surface coated with an adhesive. A cover sheet is then bonded to the adhesive coated embossed fiber board. The adhesive disclosed in this reference is not transferred off the fiber board, it is simply a discontinuous adhesive on the embossed fiber board that bonds the cover sheet to it.
None of the above cited prior art, discloses a segmented pressure sensitive adhesive transfer tape made by putting adhesive into recesses or pockets of an embossed carrier web.
Accordingly it is desirable to provide a segmented pressure sensitive adhesive transfer tape that prevents lateral flow of the adhesive segments before transfer but, permits lateral flow of the adhesive segments after transfer and bonding such that continuous adhesive bonds may be formed.
It is desirable to provide an economical method for forming and maintaining a patterned pressure sensitive adhesive during manufacturing, distribution and storage that uses ordinary coating apparatus.
It is also desirable to provide a segmented pressure sensitive adhesive transfer tape that has closely spaced, small adhesive segments with controllable thicknesses wherein the adhesive segments are readily transferable to an article or substrate.